1. Field of the Invention
The invention relates to a manufacturing method of over-current protection devices.
2. Background of the Invention
The conventional over-current protection device 10, as shown in FIG. 1, includes a current sensing element 11, an upper electrode foil 12 and a lower electrode foil 13. Because the characteristic of the resistance value of the current sensing device 11 is very sensitive to the temperature variation, such device is now widely used to protect the battery or circuit devices.
Presently, the general current sensing element 11 is formed of a conductive material with positive temperature coefficient (PTC), which includes a polymer and conductive filler. The resistance value of the PTC conductive composite material is very sensitive to temperature variation. That is, under normal operation, the resistance may remain at extremely low value in order for the circuit to operate normally. But when the over-current or over-temperature phenomenon happens, the resistance value will rise to a high resistance state instantly (at least 104 ohm), and the excess current will be eliminated reversely to achieve the object of protecting the battery or circuit devices.
The conventional manufacturing method of the over-current protection device is to first mix at least one polymer and conductive filler sufficiently and uniformly to form a PTC conductive composite material. Next, the PTC conductive composite material acting as the current sensing element are pressed with two metal foils to form a PTC plaque. Then, the PTC plaque is radiated with radioactive rays to conduct the cross-linking reaction in the conductive composite material of the PTC plaque for enhancing the electrical property. Finally, the PTC plaque is punched to form a plurality of over-current protection devices.
However, the PTC devices are prepared by stamping operation which results in temperature rise of the stamping mold and a local heating at the newly formed PTC cutting surface. After stamping, the PTC device cools down to room temperature. Since PTC device consists of metal foil and PTC conductive composite material, and the thermal expansion coefficients of the metal foil and the PTC conductive composite material are quite different from each other, the shrinkage of the metal foil and the PTC conductive composite material will be different. After a cycle of stamping heating and room temperature cooling internal stress is generated inside the PTC device and a deformation of the PTC plaque could be observed. The stress generated in the PTC conductive composite material and the metal foil may be expressed as the following equation: σ=c(αmetal−αPTTC)×ΔT, wherein σ represents the stress difference between the PTC plaque and the metal foil due to expansion and shrinkage, c is a constant, αmetal is the thermal expansion coefficient of the metal foil , αPTTC is the thermal expansion coefficient of the PTC conductive composite material, and ΔT represents the temperature difference between the metal foil and the PTC conductive composite material caused by a rise in temperature from the local heating during punching. From the above equation, the higher the temperature difference between the metal foil and the PTC conductive composite material due to punching is, the greater the stress generated inside the PTC plaque due to expansion and shrinkage, and also the larger the deformation. Therefore, after punching of the PTC plaque, stress will be generated inside the formed over-current protection device due to thermal expansion. The heated PTC conductive composite material will extrude from the edges of the upper and lower electrode foils due to expansion, and result in the warpage on the edges of the upper and lower electrode foils due to the expansion of the FTC conductive composite material, which will cause an unexpected damage on the over-current protection device.
In order to solve the described defects of the over-current protection device due to stress and heating expansion, U.S. Pat. No. 6,130,597 described a manufacturing method for the over-current protection device, which conducts a heat treatment on the formed over-current protection device after punching to compensate the defects in the over-current protection device resulting from the heating due to punching. However, the conventional method will complicate the original process along with a drawback of increasing device resistance. In addition, the conventional heat treatment method could not completely compensate for the defects resulting from internal stresses.
Following the enhanced precision of the portable electronic product, slight defects on the material will also influence the normal operation of the electronic product. Thus, it is necessary to find a solution that focuses on solving such problem.